Auto box won't change into 4th

Hi .. just got my diesel berlingo auto it's a 2001 .. it's done 55,000 miles and was sitting on a driveway since march untill I bought her.. it changes through gears ok untill it's time to go into top gear and won't shift! 40 mph and engine screaming to change but it won't! Checked transmission fluid and topped up about 500 mil ...still no joy please help!... car is perfect but for this